1. Field of the Invention
The present disclosure relates to a semiconductor device.
2. Description of the Related Art
A structure of a semiconductor device formed of a group III nitride-based compound semiconductor (e.g., a Schottky barrier diode), in which a GaN layer (GaN-FP layer) serving as a field plate is formed on an AlGaN/GaN heterostructure, has been disclosed conventionally. Further, there is also one in which an undoped GaN layer is formed between a GaN-FP layer and an AlGaN/GaN heterostructure (refer to WO 2011/162243 and Japanese Patent Application Laid-open No. 2007-134608).
With the above-described structure, two-dimensional electron gas is generated in the GaN layer at an interface between the GaN layer and the AlGaN layer, and two-dimensional hole gas is generated in the GaN-FP layer (or the undoped GaN layer) at an interface between the GaN-FP layer and the AlGaN layer.
When the two-dimensional hole gas forms an ohmic contact and is equipotential with an anode electrode, an electric field is exerted between the two-dimensional hole gas and the two-dimensional electron gas if a reverse bias is applied, and thus, the two-dimensional electron gas becomes easily depleted. Therefore, when the GaN-FP layer is formed at an end of the anode electrode, the electric field exerted on the end of the anode electrode upon reverse biasing becomes weak and thus a leakage current is expected to be suppressed.
However, with the structure in which a conductivity type of the GaN-FP layer where the two-dimensional hole gas is being generated is p-type, or with the structure in which the GaN-FP layer is not of p-type but a p-type GaN layer is formed thereon, the amount of the two-dimensional hole gas generated in the GaN-FP layer is increased. Accordingly, a concentration balance between the two-dimensional hole gas and the two-dimensional electron gas generated in the GaN layer under the AlGaN layer is lost. As a result, electrical field is concentrated at an end of the GaN-FP layer. The p-type GaN layer is doped mainly with magnesium (Mg) as a p-type dopant. As a result, mobility of holes is decreased and its difference from mobility of electrons is increased. Accordingly, concentration changes of the two-dimensional hole gas and the two-dimensional electron gas differ from each other upon switching of the semiconductor device, and the two-dimensional electron gas becomes difficult to be depleted, whereby electrical field is concentrated at the end of the GaN-FP layer. As a result, voltage endurance of the semiconductor device is decreased and leakage current is not effectively suppressed.
Accordingly, there is a need to provide a semiconductor device in which reduction in its voltage endurance is suppressed and its leakage current is effectively suppressed.